I. Field of the Invention
The present invention relates to infrared sensor signal processors. More specifically, the present invention relates to a novel and improved high gain bandpass amplifier for conditioning signals received from a passive infrared sensor. II. Background Art
The advances in the development of solid state infrared sensors, namely crystal sensors, have reached a point where their cost has fallen sufficiently for consideration for applications in moderate cost consumer products. Accordingly, interest has grown in the development of electronic circuitry to most economically utilize these sensors in minimal cost consumer products. At the present time, the most sensitive of these sensors is lithium titinate crystals. Such crystal sensors provide a maximum usable output signal on the order of a few hundreds of microvolts in response to a human body moving within a few meters of the sensor. Although these particular sensors are extremely sensitive to infrared radiation including human body heat, they are relatively expensive. Furthermore, these sensors are extremely sensitive to heat and, therefore, are not adapted to withstand conventional industrial wavesoldering techniques used in the high-volume fabrication of consumer devices.
In the entire class of infrared sensors, there exists one particular sensor type which falls within the manufacturing and cost requirements of consumer goods. This sensor type is the ceramic sensor. Ceramic sensors are not only less expensive than crystal sensors, but are also sufficient-y insensitive to temperature extremes. This insensitivity to heat readily permits their usage in circuit designs which use economical wavesoldering techniques. As such, the use of ceramic infrared sensors realize a cost-of-use within a cost range adaptable to low cost consumer products.
Although the cost and temperature ruggedness factors of the ceramic sensors are advantageous over other types of sensors, certain disadvantages exist. The primary disadvantage of using ceramic sensors is their low signal level output. Ceramic sensors typically provide a maximum signal on the order of a few tens of microvolts in response to a human body moving within a range of a few meters from the sensor. In addition, the peak signal level may last from a second to a few hundred thousandths of a second.
In the typical low cost consumer product, unshielded plastic housings must be used to realize cost savings. The use of unshielded plastic housings means that the electronic design must be inherently capable of reliably responding to the microvolt level shifts of the ceramic sensor. Furthermore, the electronic design must simultaneously ignore a full range of ambient electrical noise signal which are typically several orders of magnitude larger than the sensor signal.
In fabricating a low cost consumer product, it is important that the electronic circuitry be designed such that there exist a low part count and low cost associated with the design to be competitive in the consumer market. It is well recognized that part cost, part count and part size along with assembly costs are exceeding-y critical in consumer product designs. Part and assembly costs are both affected by the electrical and physical size of the components, with electrically larger parts typically costing more and occupying more space throughout the manufacturing process as well as within the product itself. It is, therefore, economically desirable that any consumer product be implemented with the smallest size as well as the fewest number of values of components as is possible. The market success of a low cost consumer product incorporating leading edge technology, such as ceramic infrared sensors, requires minimal electronic systems to support the technology.
It is also well known that electronic components may be obtained in practical)y any desired value. However, when standard value parts are used, costs may be minimized and the availability of the part for production is maximized. It is also a consideration in the fabrication of consumer products to utilize the most cost and performance effective technology available for implementing high volume production.
In using ceramic sensors, the output signal from the sensor must be filtered and amplified for later processing. Conventionally, a low pass filter designed for operation in a frequency range around 1 Hz, require capacitors in the 2-50 microfarad range. Accordingly, capacitors in this range are typically large and expensive. In addition, to achieve more than a 6 Db per octave slope, second or higher order filters are needed which use several of the large value capacitors.
In amplifying low level signal, the typical amplifiers used are operational amplifiers. However, conventional operational amplifiers vary in the level of input offset voltages and currents. Amplifiers having extremely low levels of offset voltages and currents are typically found only in expensive selected units. Amplifiers available at costs compatible with low cost consumer products tend to exhibit input offset voltages and currents larger than signal levels that are output by a ceramic sensor. Furthermore, amplifiers used in low cost consumer products tend to exhibit equally large lot-to-lot variations. Conventional amplifier designs used in extremely low frequency, low level signal applications previously incorporated offset adjustments. These adjustments may be either made by incorporating either manual external offset adjustment potentiometers or auto-zeroing circuitry to compensate for the large output offset which result when high gain circuitry is used. In either case the additional circuitry required or manufacturing time to adjust the product can add additional cost to the product.
Low cost consumer products utilizing ceramic sensors and their related electronics may involve the use of power control circuitry. For example, in one application of the present invention, the sensor and associated electronics is used to control a thyristor/triac which controls the illumination of an incandescent lamp. Lamp loads may reach up to 300 watts with power provided directly off a 120 volt, 60 Hz line. The use of power control circuitry typically creates large spike-like transients with each turn-on of the controlled device. In many consumer products, it is necessary for the sensor and the sensor signal conditioning electronics to be mounted on a circuit board within a housing within a few centimeters of the switching triac and the AC power line. In such situations, the close proximity of the power and power control electronics to be sensor electronics can result in noise problems, should the sensor electronics be not sufficiently insensitive to the noise.